As users experience the convenience of wireless connectivity, they are demanding increasing support. Typical applications over wireless networks include video streaming, video conferencing, distance learning, etc. Because wireless bandwidth availability is restricted, quality of service (QoS) management is increasingly important in 802.11 networks.
The original 802.11 media access control (MAC) protocol was designed with two modes of communication for wireless stations. The first mode, Distributed Coordination Function (DCF), is based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), sometimes referred to as “listen before talk.” A station waits for a quiet period on the network and then begins to transmit data and detect collisions. The second mode, Point Coordination Function (PCF), supports time-sensitive traffic flows. Using PCF, wireless access points (APs) periodically send beacon frames to communicate network identification and management parameters specific to the wireless local area network (WLAN). Between sending beacon frames, PCF splits time into a contention period where the stations implement a DCF protocol, and a contention-free period where an AP coordinates access by the various stations based on QoS requirements.
Because DCF and PCF do not differentiate between traffic types or sources, IEEE proposed enhancements to both coordination modes to facilitate QoS. These changes are intended to fulfill critical service requirements while maintaining backward-compatibility with current 802.11 standards.
Enhanced Distributed Channel Access (EDCA) introduces the concept of traffic categories. Using EDCA, stations try to send data after detecting that the medium is idle for a set time period defined by the corresponding traffic category. A higher-priority traffic category will have a shorter wait time than a lower-priority traffic category. While no guarantees of service are provided, EDCA establishes a probabilistic priority mechanism to allocate bandwidth based on traffic categories.
The IEEE 802.11e EDCA standard provides QoS differentiation by grouping traffic into four access classes (ACs), i.e. voice, video, best effort and background. Each transmission frame from the upper layers bears a priority value (0-7), which is passed down to the MAC layer. Based on the priority value, the transmission frames are mapped into the four ACs at the MAC layer. The voice AC has the highest priority; the video AC has the second highest priority; the best effort AC has the third highest priority; and the background AC has the lowest priority. Each AC has its own transmission queue and its own set of medium access parameters. Traffic prioritization uses the medium access parameters—AIFS interval, contention window (CW, CWmin and CWmax), and transmission opportunity (TXOP)—to ensure that a higher priority AC has relatively more medium access opportunity than a lower priority AC.
Generally, the arbitration interframe space (AIFS) is the time interval that a station must sense the medium to be idle before invoking a backoff mechanism or transmission. A higher priority AC uses a smaller AIFS interval. The contention window (CW, CWmin and CWmax) indicates the number of backoff time slots until the station can attempt another transmission. The contention window is selected as a random backoff number of slots between 0 and CW. CW starts at CWmin. CW is essentially doubled every time a transmission fails until CW reaches its maximum value CWmax. Then, CW maintains this maximum value until the transmission exceeds a retry limit. A higher priority AC uses smaller CWmin and CWmax. A lower priority AC uses larger CWmin and CWmax. The TXOP indicates the maximum duration that an AC can be allowed to transmit transmission frames after acquiring access to the medium. To save contention overhead, multiple transmission frames can be transmitted within one acquired TXOP without additional contention, as long as the total transmission time does not exceed the TXOP duration.
To reduce the probability of two stations colliding, because the two stations cannot hear each other, the standard defines a virtual carrier sense mechanism. Before a station initiates a transaction, the station first transmits a short control frame called RTS (Request To Send), which includes the source address, the destination address and the duration of the upcoming transaction (i.e. the data frame and the respective ACK). Then, the destination station responds (if the medium is free) with a response control frame called CTS (Clear to Send), which includes the same duration information. All stations receiving either the RTS and/or the CTS set a virtual carrier sense indicator, i.e., the network allocation vector (NAV), for the given duration, and use the NAV together with the physical carrier sense when sensing the medium as idle or busy. This mechanism reduces the probability of a collision in the receiver area by a station that is “hidden” from the transmitter station to the short duration of the RTS transmission, because the station hears the CTS and “reserves” the medium as busy until the end of the transaction. The duration information in the RTS also protects the transmitter area from collisions during the ACK from stations that are out of range of the acknowledging station. Due to the fact that the RTS and CTS are short, the mechanism reduces the overhead of collisions, since these transmission frames are recognized more quickly than if the whole data transmission frame was to be transmitted (assuming the data frame is bigger than RTS). The standard allows for short data transmission frames, i.e., those shorter than an RTS Threshold, to be transmitted without the RTS/CTS transaction.
With these medium access parameters, EDCA works in the following manner:
Before a transmitting station can initiate any transmission, the transmitting station must first sense the channel idle (physically and virtually) for at least an AIFS time interval. If the channel is idle after an initial AIFS interval, then the transmitting station initiates an RTS transmission and awaits a CTS transmission from the receiving station.
If a collision occurs during the RTS transmission or if CTS is not received, then the transmitting station invokes a backoff procedure using a backoff counter to count down a random number of backoff time slots selected between 0 and CW (initially set to CWmin). The transmitting station decrements the backoff counter by one as long as the channel is sensed to be idle. If the transmitting station senses the channel to be busy at any time during the backoff procedure, the transmitting station suspends its current backoff procedure and freezes its backoff counter until the channel is sensed to be idle for an AIFS interval again. Then, if the channel is still idle, the transmitting station resumes decrementing its remaining backoff counter.
Once the backoff counter reaches zero, the transmitting station initiates an RTS transmission and awaits a CTS transmission from the receiving station. If a collision occurs during the RTS transmission or CTS is not received, then the transmitting station invokes another backoff procedure, possibly increasing the size of CW. That is, as stated above, after each unsuccessful transmission, CW is essentially doubled until it reaches CWmax. After a successful transmission, CW returns to its default value of CWmin. During the transaction, the station can initiate multiple frame transmissions without additional contention as long as the total transmission time does not exceed the TXOP duration.
The level of QoS control for each AC is determined by the combination of the medium access parameters and the number of competing stations in the network. The default EDCA parameter values used by non-AP QoS stations (QSTAs) are identified in FIG. 1. A TXOP_Limit value of 0 indicates that a single MAC service data unit (MSDU) or MAC protocol data unit (MPDU), in addition to a possible RTS/CTS exchange or CTS to itself, may be transmitted at any rate for each TXOP.
Hybrid Coordination Function Controlled Channel Access (HCCA) uses the concepts of PCF, but essentially substitutes the protocols of DCF with the improved protocols of EDCA. That is, using HCCA, APs periodically send beacon frames. Between beacon frames, HCCA splits time into a contention period and a contention-free period. During the contention period, EDCA protocols are used. During the contention-free period, the AP coordinates access by the various stations based on QoS requirements.
Networks implementing AP coordinated access, e.g., PCF and HCCA, require complex implementation and substantial overhead. Accordingly, network designers prefer to implement simpler networks using distributed wireless access, e.g., DCF or EDCA. However, because stations implementing distributed wireless access, e.g., DCF, EDCA, etc., seize access to the wireless medium without a coordinator, collisions occur (especially when many stations are competing for access to a wireless channel). Further, different ACs do not preclude the chance of a low priority packet colliding with a higher priority packet. Accordingly, systems and methods are needed that can reduce the risk of collisions in networks that implement distributed wireless access and particularly when implementing multiple access classes, e.g., using EDCA.
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